The present invention relates to lasers, and more particularly, to a rare earth element doped laser with longitudinal mode selection. The doped laser is implemented in an optical transmission medium such as an optical fiber or silica waveguide. The rare earth material can comprise, for example, Erbium, Neodymium or Praseodymium.
Applications for optical fiber technology are expanding at a rapid pace. Telecommunication, sensors, medical and video transmission can all take advantage of optical technology, particularly where virtually unlimited bandwidth and low attenuation are beneficial. Cable television systems are one example where optical fiber technology is providing efficient and economical alternatives to prior coaxial cable distribution schemes.
Many applications for fiber optic technology would be more cost effective if higher power lasers operating in the 1.3 or 1.5 micron (".mu.m") region were available. It has been demonstrated that a Er.sup.3+ fiber laser can produce high levels of power. See, e.g., M. S. O'Sullivan, et al, "High Power Narrow Linewidth Erbium-Doped Fiber Laser", CLEO 1989, TUP3, pp. 134-136. A drawback of an erbium doped fiber laser as taught in the referenced article is that it has multiple longitudinal modes. Some applications, such as the transmission of complex video signals over an optical fiber in a cable television distribution network or the like, require the laser to operate at only a single longitudinal mode or, at most, only a few such modes separated sufficiently in optical frequency. Otherwise, beating will occur (e.g., between optical longitudinal modes that fall in the radio frequency domain) resulting in unacceptable levels of noise in the RF band of interest. Those modes of laser operation that are close enough together to be on the order of RF frequencies (i.e., 5.75 MHz to 550 MHz or higher for the cable television spectrum) must be suppressed in order to use such lasers for CATV applications. After suppression, the only remaining modes will not produce beat frequencies in the RF domain. Multiple longitudinal operation may also exhibit excess intensity noise (RIN) owing to the mode partition noise as the various modes turn on and off. As is well known in the art, the actual modes present in a given laser are determined by the geometry of the laser cavity, laser gain spectrum and frequency selective elements in the cavity.
It would be advantageous to provide a rare earth element doped laser in which all but one mode is suppressed. Those skilled in the art will appreciate that additional modes can remain as long as they are far enough apart in frequency so that beating does not occur. Such a laser should be operable without producing interfering beats in the RF spectrum. The resultant energy from the laser must comprise a clean optical carrier for signal transmission.
The present invention provides a laser having the aforementioned advantages.